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An Introduction to Fluoroscopy Safety

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An Introduction to Fluoroscopy Safety An Introduction to Fluoroscopy Safety Charles M. Anderson MD, PhD Edwin M. Leidholdt, Jr. PhD Introduction Many physicians assume fluoroscopy is inherently safe technology. Yet each year several patients in the U.S. suffer permanent skin damage from fluoroscopic procedures, requiring surgical correction. In addition, radiation has the potential to induce cancer. This manual describes the techniques one can use to reduce fluoroscopic radiation dose to patients, while maintaining acceptable image quality. If you intend to operate a fluoroscope, we suggest you read this manual, and then: Ask someone to demonstrate the controls of the equipment you will be using. Have an experienced operator observe your first cases. Participate in a QA program, with the goal of minimizing dose without impairing clinical outcome. Ensure that the equipment you are using has been inspected by a medical physicist according to State or Federal regulations, and that the room shielding has been similarly tested. Acknowledgements This manual is based on a Department of Veterans Affairs course developed by Drs. Anderson and Leidholdt. We thank Fred A. Mettler Jr., MD, MPH and Donald P. Frush, MD FACR, FAAP for reviewing the manuscript. Disclaimer The Department of Veterans Affairs is not responsible for the contents of this manual. Even if one employs dose sparing techniques as described in this manual, a patient could be injured. Some of the techniques described here are vendor and model specific. Your equipment may operate differently. Consult the operating manual or your vendor’s technical representative. Version: August 19, 2013 An Introduction to Fluoroscopy Safety 2 Table of Contents Page Chapter 2 Introduction 4 Measuring Dose 9 Biological Effects of Radiation 16 Fluoroscope Technology and Image Quality 27 Minimizing Dose to the Patient 31 Pediatric Fluoroscopy 33 Minimizing Dose to the Practitioner 39 Resources An Introduction to Fluoroscopy Safety 3 Measuring Dose A discussion of fluoroscope settings and their effect on x-ray dose will be more meaningful once we have defined x-ray dose and explained how it is measured. Motivation for Measuring Radiation Dose There are practical reasons why you should understand dose units and take note of the radiation dose of each procedure you perform. If you know the typical dose metrics for procedures you perform, you can conduct an appropriate risk/benefit conversation with your patients before the procedure begins. If you keep track of how much dose your patients have received during a procedure, you can determine when the benefits of continuing no longer exceed the risks. If you note the measurements of dose at the end of a procedure, you can plan for follow-up care if necessary. If you compare the dose metrics of your procedures to those of similar procedures that your colleagues or other practices have performed, you can improve your dose sparing techniques. Quantities and Units Absorbed dose The absorbed dose is the amount of radiation energy absorbed by tissue per mass of tissue. Skin typically receives the highest absorbed dose. Peak Skin Dose The peak skin dose is the absorbed dose at the skin location that has received the highest dose. This quantity is used to predict a skin injury. Entrance and Exit Skin Dose X-rays are progressively absorbed as they pass through the body. For every 4 cm of tissue, the beam strength is reduced by about one-half. As a result, the dose received where the beam enters the body is much higher than the dose where it exits. For a typical adult abdomen, and depending on the energy of the x-ray beam, the entrance dose can be about 100 times greater than the exit dose.1 Effective Dose Effective dose is an approximate measure of potential harm from cancer. A procedure directed at the chest may be more likely to induce cancer than a procedure directed at an extremity, because lungs are more susceptible to cancer than is muscle. Effective dose takes these differences in cancer risk into account so that procedures can be compared in terms of their cancer potential. 1 Wagner LK and Archer BR (2004). Minimizing risks from fluoroscopic x-rays. The Woodlands, TX: R.M. Partnerships. An Introduction to Fluoroscopy Safety 4 Gray Fluoroscopes display dose in units of gray (Gy). A gray is the amount of radiation energy deposition equal to one joule absorbed per kilogram of tissue. The Gy replaces the traditional unit of rad, whereby 1 Gy equals 100 rad. Sievert The sievert (Sv) is similar to Gy but takes into account the potential ability of the radiation to cause a biological effect, primarily cancer. The Sv replaces the traditional unit of rem, whereby 1 Sv equals 100 rem. The rem is still used in occupational health. Absorbed skin dose is measured in Gy. Effective dose is measured in Sv. Personal dosimetry badges often report effective dose in units of mrem. Typical Doses Mettler et. al.2 have published typical procedure effective doses for adults, which is the main source for the table below. The effective dose of a simple fluoroscopic exam is more than 100 times greater than that of a chest x-ray, while the effective dose of a complex fluoroscopically guided intervention can be thousands of times greater than that of a chest x-ray. Procedure Effective Dose (mSv) Bone densitometry (DXA) 0.001 Dental Intraoral X-ray 0.005 PA Chest X-ray 0.02 Mammogram 4-views 0.4 Abdomen X-ray 0.7 Barium Swallow 6 Barium Enema 8 CT Head, each series 2 CT Abdomen, each series 8 PET/CT (F-18 FDG) 14 Endoscopic Retrograde Cholangiopancreatography 4 Coronary Angiography 2-16 Coronary Angioplasty or Radiofrequency Ablation 7-57 Transjugular Intrahepatic Portosystemic Shunt Placement 20-180 Dose Estimation Skin dose can be estimated by a variety of means, with varying accuracy. One can place dosimeters directly on the patient’s skin, one can use the dose estimate provided by the fluoroscope, or one can note how long the beam is on. One can further refine these estimates by taking into account how the dose was distributed over various skin sites. 2 Mettler, FA Jr, Huda, W, Yoshizumi, TT, Mahesh, M, Effective doses in radiology and diagnostic nuclear medicine: A catalog, Radiology 2008;248:254-263. An Introduction to Fluoroscopy Safety 5 Direct Measurement Several types of skin dosimeters are available to measure entrance dose directly. Some measure dose in real time and can trigger an alarm when dose thresholds have been exceeded. Others can only be read at the end of the procedure. Depending on where the dosimeter is placed, it may or may not measure the actual peak skin dose. Dosimeters are the most accurate measurement tool, but they are cumbersome and not necessary for most fluoroscopic procedures. Estimation by the Fluoroscope Based on the fluoroscope settings and beam-on time, the instrument itself provides an estimate of dose. Because the fluoroscope does not know the actual position of the patient, the dose is not always accurate. To understand how the instrument makes this estimation one must understand the inverse square law. Inverse Square Law As a radiation beam emerges from the x-ray tube it diverges, covering a wider area with increasing distance from the tube, but with decreasing beam strength. In the illustration below on the left, x-rays are directed upwards from the source. At position “A” the width of the beam is, for example, 10 cm, the area of the beam is 100 cm2, and the dose rate is 30 mGy/min. At “B” which is twice the distance from the source, the width of the beam is 20 cm, the area is 400 cm2 and the dose rate is 7.5 mGy/min. At B the area is four times larger than at A, and the dose rate is reduced to one-fourth. The dose rate is proportional to the inverse of the square of the distance from the source. In the graph on the right the dose rate in this example is calculated out to a distance of 100 cm from the source. Note how dramatically the dose rate rises as one approaches the x-ray tube. Interventional Reference Point Because of the inverse square law, the fluoroscope must know the distance from the x-ray source to the patient’s skin in order to estimate the dose rate. The fluoroscope assumes the entrance skin is at the “reference point,” also called the interventional reference point (IRP) which for cardiac and interventional fluoroscopes is commonly 15 cm from the isocenter to the source. The isocenter is on An Introduction to Fluoroscopy Safety 6 the axis of rotation of the “C-arm” that holds the x-ray source and detector, which is typically close to the center of the patient. The axis of rotation of the C-arm is depicted as a dashed line in the illustration to the left. The isocenter lies on the rotational axis, between the source and detector. In the illustration on the left below, the patient chest cross-section is represented by the light blue ellipse. The isocenter is at the tail of the white arrow and the IRP is 15 cm closer to the x- ray source at the head of the arrow. In this example the IRP is exactly at the skin. In the illustration on the right, the estimated entrance skin location is erroneous for an obese patient. If the patient is obese or if the source is moved closer to the patient, the skin dose will be underestimated. Air Kerma The dose quantities that are calculated by the fluoroscope are the “air kerma” and the “kerma area product.” Air kerma is the amount of energy per unit mass absorbed by air at the assumed location of the skin, usually expressed in mGy.
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